I have been playing with my friend who doesn't own lightfall and they have done 4 vex strike forces in a row and haven't received new exotics at all. They have always been duplicates. Meanwhile every time I do it (and I own lightfall) I get exotics I don't have yet (lost sector exotics). If this is because they don't own lightfall, this is ridiculous! It is a free event for everyone. I can understand it not dropping the new exotics (from lightrfall), but the old ones in lost sectors? Feels like another aggressive monetization or a bug. Either way needs to be clarified or fixed.
You need to own the yearly expansion an exotic was released on the platform where you are playing in order for that exotic to drop. The system has worked this way since it was introduced 2-3 years ago.